Re: [PATCH 2/2] x86/vpci: refuse to map BARs at position 0

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, May 22, 2025 at 03:22:53PM +0100, Andrew Cooper wrote:
> On 22/05/2025 3:03 pm, Roger Pau Monne wrote:
> > A BAR at position 0 is not initialized (not positioned).  While Xen could
> > attempt to map it into the p2m, marking it as mapped will prevent dom0 to
> > change the position of the BAR, as the vPCI code has a shortcomming of not
> 
> Minor grammar point.  "prevent dom0 from changing".
> 
> > allowing to write to BAR registers while the BAR is mapped on the p2m.
> >
> > Workaround this limitation by returning false from pci_check_bar() if the
> > BAR address is 0, thus causing the bar->enabled field to also be set to
> > false and allowing bar_write() to change the BAR position.
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/pci.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/xen/arch/x86/pci.c b/xen/arch/x86/pci.c
> > index 26bb7f6a3c3a..39fd5a16a4aa 100644
> > --- a/xen/arch/x86/pci.c
> > +++ b/xen/arch/x86/pci.c
> > @@ -101,6 +101,15 @@ int pci_conf_write_intercept(unsigned int seg, 
> > unsigned int bdf,
> >  
> >  bool pci_check_bar(const struct pci_dev *pdev, mfn_t start, mfn_t end)
> >  {
> > +    /*
> > +     * Refuse to map BARs at position 0, those are not initialized.  This 
> > might
> 
> "0, as they are not"
> 
> > +     * be required by Linux, that can reposition BARs with memory decoding
> 
> "Linux, which may reposition".
> 
> Otherwise, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Thanks, since this is not blocking the CI right now I will probably
wait a bit to gather more feedback.

Roger.